Support SAML authentication (#25165)

Closes https://github.com/go-gitea/gitea/issues/5512

This PR adds basic SAML support
- Adds SAML 2.0 as an auth source
- Adds SAML configuration documentation
- Adds integration test:
- Use bare-bones SAML IdP to test protocol flow and test account is
linked successfully (only runs on Postgres by default)
- Adds documentation for configuring and running SAML integration test
locally

Future PRs:
- Support group mapping
- Support auto-registration (account linking)

Co-Authored-By: @jackHay22

---------

Co-authored-by: jackHay22 <jack@allspice.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: KN4CK3R <admin@oldschoolhack.me>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: morphelinho <morphelinho@users.noreply.github.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>
Co-authored-by: Yarden Shoham <git@yardenshoham.com>
Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: silverwind <me@silverwind.io>

.github/workflows/pull-db-tests.yml

@@ -37,6 +37,14 @@ jobs:
           MINIO_ROOT_PASSWORD: 12345678
         ports:
           - "9000:9000"
+      simplesaml:
+        image: allspice/simple-saml
+        ports:
+          - "8080:8080"
+        env:
+          SIMPLESAMLPHP_SP_ENTITY_ID: http://localhost:3002/user/saml/test-sp/metadata
+          SIMPLESAMLPHP_SP_ASSERTION_CONSUMER_SERVICE: http://localhost:3002/user/saml/test-sp/acs
+          SIMPLESAMLPHP_SP_SINGLE_LOGOUT_SERVICE: http://localhost:3002/user/saml/test-sp/acs
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-go@v5